1. Field of the Invention
The present invention relates to a receiving end of a low noise amplifier, and more particularly to a circuit for electrostatic discharge protection and input impedance matching of the low noise amplifier.
2. Description of the Related Art
Generally, a low noise amplifier having improved noise characteristics is used at a radio frequency (RF) receiver end to amplify a received RF signal. In addition, the low noise amplifier is used for impedance matching between internal elements of the low noise amplifier and an external circuit.
A degeneration inductor is widely used for impedance matching of a low noise amplifier. However, when a degeneration inductor is used for impedance matching of a low noise amplifier, noise figure may be deteriorated due to parasitic capacitance of an electrostatic discharge protection circuit and a bonding pad of the low noise amplifier so that implementing an impedance matching low noise amplifier becomes difficult.
FIGS. 1 and 2 show equivalent circuit diagrams illustrating a receiving end of a conventional low noise amplifier 10.
Referring to FIGS. 1 and 2, the conventional low noise amplifier 10 includes a gate pad PG1, parasitic capacitors C1 and C2, an electro-static discharge protection unit D1, an input transistor M1, a source pad PS1 and a source degeneration inductor L1.
A received RF signal is input to the input transistor M1 through the gate pad PG1 and buffered by the input transistor M1 to be amplified. The electro-static discharge protection unit D1 is used to protect an RF receiver chip RFIC from an external electro-static discharge. As shown in FIG. 2, the parasitic capacitor C1 may be formed between the gate pad PG1 and a substrate SUBSTRATE of the chip RFIC. In addition, the parasitic capacitor C2 may also be formed between a source electrode and a gate electrode of the input transistor M1 during the manufacturing process of a semiconductor.
The source degeneration inductor L1 is used to remove a capacitance component of an impedance value (imaginary impedance) so that only a real impedance component will remain. The source degeneration inductor L1 may be implemented using a bonding wire to couple the source pad PS1 to an external circuit.
The input impedance of the RF receiver chip RFIC seen by the gate pad PG1 may be represented as a function of the capacitance of capacitor C2, the inductance of the source degeneration inductor L1 and the transconductance of the input transistor M1. To provide a target (desired) impedance value having only a real component required by a system, for example, 50 Ω, the capacitance value of the capacitor and the inductance value of the source degeneration inductor L1 may be adjusted to cancel each other. In addition, a matching inductor L2, which may be implemented in a separate chip and coupled to the gate pad PG1, may be used for ease of impedance matching.
As described above, except the capacitance C2 formed between the gate and source electrodes of the input transistor M1, the parasitic capacitance C1 between the substrate and the gate pad PG1 and the parasitic capacitance (not shown) between the substrate and the electrostatic discharge protection unit D1 are not used for impedance matching of the low noise amplifier, causing difficulty in the impedance matching. Moreover, the parasitic capacitance may lower the gain of the low noise amplifier and result in degradation of the noise figure.